This invention relates to an electrical apparatus for providing an indication of the level of a liquid contained in a container having walls with good heat conducting properties. In particular, but not exclusively, the apparatus is intended to provided a visual and/or audible indication of the level of liquid gas, e.g. liquid propane or liquid butane, in a metal walled, valved container of the kind used for camping or space heating purposes.
In EP-A-No. 119072 there is described a liquid level indicator for indicating the level of contained liquid gas n a cylinder which comprises a number of temperature-sensing strips each including at least one thermochromatic liquid crystal composition. In use the indicator is applied to the outside of the cylinder for liquid gas. As gas is withdrawn from the cylinder, liquid gas within the cylinder vaporises and a temperature difference or gradient is established in the wall of the cylinder at the level of the contained liquid gas. This temperature gradient is detected by the temperature-sensing strips, at least one of which changes colour at the level of the contained liquid gas. The vaporisation of the liquid gas within the cylinder, creating the temperature gradient in the cylinder walls at the level of the contained liquid gas, occurs naturally as gas is withdrawn from the cylinder. However a temperature gradient may also occur either under normal storage conditions in which ambient air temperature fluctuates or by dowsing the container with hot or cold water.
It is also known from U.S. Pat. No. 3,161,050, U.S. Pat. No. 4,065,967 and U.S. Pat. No. 4,307,606 to use electrical apparatus for detecting the level of material in a tank or other container. In this respect, U.S. Pat. No. 3,161,050 employs a continuously energised heater mounted on the outside of a container to establish a heated zone along the height of the container and a motorised unit movable vertically up and down the container for detecting the temperature of the container wall at spaced unequal distances from the heater.
U.S. Pat. No. 4,065,967 discloses a silo level indicating system having a plurality of sensor units positioned at vertically spaced apart fixed positions on the outside wall of the silo. The uppermost sensor unit is a reference sensor unit and is positioned so as always to be above the level of material contained in the silo. The remaining sensor units detect the temperature of the silo wall at lower levels of the silo and compare these temperatures with that detected by the reference sensor unit. The existence of a significant temperature differential between the temperatures detected by the reference sensor unit and a compared sensor unit indicates the presence of material at or above the compared sensor unit.
U.S. Pat. No. 4,307,606 discloses apparatus for indicating the location of a thermal transition zone between heated fluid and unheated fluid in a storage tank. As in U.S. Pat. No. 4,065,967, the apparatus comprises a plurality of vertically spaced apart sensor units each designed to detect the temperature of the wall of the storage tank at the level of the sensor unit. It is necessary to provide a plurality of sensor units attached in fixed positions to the storage tank to obtain an effective indication of the level of fluid contained in the storage tank.